If it's a variable speed fan then it's using a triac based circuit to chop the 120 vac so the fan runs slower, the humming is probably the windings, you could take it apart and try some silicone or hot glue on the open spaces. Or you could take to back to HD.

One could have a bent or rusted shaft or a funky bearing, which presents a different load. If the shaft is chromed (unlikely) the plating may be uneven or coming off creating a different load. Do they have the same diameter and length of ducts? Are you sure that the damper is opening? Are the ducts actually unobstructed (bird's nest in one)?

One could have a bent or rusted shaft or a funky bearing, which presents a different load. If the shaft is chromed (unlikely) the plating may be uneven or coming off creating a different load. Do they have the same diameter and length of ducts? Are you sure that the damper is opening? Are the ducts actually unobstructed (bird's nest in one)?

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There's no damper and no ductwork. This is a 5-blade, 3-speed ceiling fan with dimable light. It is remote-controlled from a hard-wired switch on the wall to a receiver hidden in the mounting bracket.